Heat-resistant to low chromium-nickel alloy steel for large forgings

ABSTRACT

A heat-resistant alloy steel which is basically a 1 percent chromium-molybdenum-vanadium steel with the addition of from 0.03 to 0.15 percent by weight niobium and from 0.002 to 0.010 percent by weight boron. High hardenability and good high temperature ductility and creep properties are developed by austenitising the steel in the range 950* C. to 980* C., hardening by cooling to obtain a mainly bainitic structure, and tempering in the range 650* C. to 720* C. for at least 12 hours.

United States Patent [72] lnventors Kenneth Arnold Ridal; [56] R f ren Cited golgnlzlzeCann, both of Yorkshire, England UNITED STATES PATENTS '5 gm" 3 19 1968 2,880,085 3 1959 Kirkby 75 1 28.6 la Sept 1971 2,968,549 1/1961 Brady 75/l28.6 [73] Assignec English Steel Corporation Limited Primary Examiner-Hyland Bizot Sheffield, England Attorney-Stevens, Davis, Miller & Moshcr [32] Priority June 29, 1967 [33] Great Britain [31 30166/67 ABSTRACT: A heat-resistant alloy steel which is basically a l 54] EAT RESISTANT To Low CHROMIUM NICKEL percent ehromium-molybdenum-vanadium steel with the ad- ALLOY STEEL FOR LARGE FORGINGS dition of from 0.03 to 0.15 percent by weight niobium and from 0.002 to 0.010 percent by weight boron. High hardcnability and good high temperature ductility and creep properties are developed by austenitising the steel in the range 950 C. to 980 C., hardening by cooling to obtain a mainly bainitic structure, and tempering in the range 650 C. to 720 C. for at least 12 hours.

HEAT-RESISTANT TO LOW CHROMlUM-NICREL ALLOY STEEL FOR LARGE EORGINGS This invention relates to heat-resistant alloy steels and is concerned with producing agenerally improved steel of this nature having good resistance to high temperature creep combined with good mechanical properties, for use in large forgings.

BACKGROUND OF THE lNVENTlON Conventional 1 percent ehromium-molybdenum-vanadium steels when utilized for large forgings from which high and intermediate pressure steam turbine rotors having a mass equivalent to or greater than that of a 24 inch diameter cylinder when hardened, are to be manufactured, frequently give rise to rotors with inferior high temperature creep properties. Previous attempts to provide a steel with good high temperature creep properties for the manufacture of such turbine rotors have resulted in steels with the required high temperature creep properties but with poorer ductility and hardenability. With such heretofore known steels it has also been difficult to ensure uniform properties throughout heavy forgings.

it is accordingly an object of this invention to provide a new and improved heat-resistant alloy steel which possesses good mechanical properties, creep strength and ductility.

it is another object of this invention to provide a new and improved heat-resistant alloy steel which has good intrinsic hardenability, high temperature ductility and high temperature creep properties.

It is a further object of this invention to provide a new and improved heat-resistant alloy steel which when utilized for the manufacture of large heavy forgings ensures that good high temperature creep and ductility properties are maintained throughout the forging.

SUMMARY OF THE INVENTION The foregoing objects are accomplished by providing a heat-resistant alloy steel containing by weight from 0.20 to 0.30 percent carbon, from to 0.30 percent silicon, from 0.4 to 1.0 percent manganese, from 0.4 to 1.0 percent nickel,

from 0.7 to 1.4 percent chromium, from 0.5 to 1.5 percent molybdenum, and from 0.25 to 0.40 percent vanadium, and further containing by weight from 0.03 to 0.15 percent niobium and from 0.002 to 0.010 boron to improve the hardenability, high temperature ductility and high temperature creep properties of the steel, the balance except for impurities and incidental constituents which include from O to 0.040 percent by weight sulfur and from 0 to 0.040 percent by weight phosphorus, being iron. The steel of the invention is heat treatable by austenitising in a temperature range 950 C. to 980 C., hardening by cooling to obtain a mainly bainitic structure, and tempering in a temperature range 650 C. to 720 C. for at least 12 hours.

DESCRIPTION OF PREFERRED EMBODIMENTS Constituent Percentage of total weight Carbon from 0.22 to 0.26

Silicon up to a maximum of 0.30

Manganese from 0.5 to 0.7 Nickel from 0.5 to 0.7 Chromium from 0.9 to 1.1 Molybdenum from 0.65 to 0.85 Vanadium from 0.30 to 0.35 Niobium from 0.05 to 0.09

Boron from 0.004 to 0.008.

, 1n the foregoing range of constituents for a steel produced according to the invention, the balance of the constituents is iron, except for impurities and incidental constituents, such as sulfur which is present in an amount not exceeding 0.015 percent of the total weight, and phosphorous, which is also present in an amount not exceeding 0.15 percent of the total weight.

in steels produced in accordance with the invention the content of carbon, hardening elements, and carbide formers has been balanced to give an optimum combination of creep strength and mechanical properties. Furthermore the careful control of these elements, particularly carbon, vanadium and niobium and boron allows such properties to be obtained throughout heavy forgings. The selection in a steel produced according to the invention, of an optimum composition, particularly of carbon, manganese, nickel, chromium, molybdenum and vanadium, plus the addition of a controlled amount of niobium and boron allows tensile properties of 37 tons per square inch at 0.2 percent proof stress and 45 tons per square inch ultimate tensile strength to be obtained throughout the section of large forgings made from such stccl. These properties are the minimum at any point within the forging, and in practice properties superior to these will normally be obtained. Additionally a steel produced according to the invention is particularly resistant to high temperature creep, and samples prepared from any part of a forging made from such a steel would withstand 1,000 hours at 550 C., and 7 tons per squareinch, and exhibit a total plastic strain of less than 0.1 percent;

The foregoing properties are developed by austenitising a forging produced from a steel of the invention in the range 950 to 980 C., and subsequently hardening the forging by cooling in air, steam, water-mist or oil so as to obtain a mainly bainitic structure. The desired properties are then obtained by tempering the hardened steel in the range of from 650 to 720 C. for at least 12 hours. Optimum properties are usually obtained if the tempering treatment is carried out in the range of from 690to 710 C.

It is to be noted that similar properties may be obtained by using steels of composition different to the steels produced in accordance with the present invention, or by modifying the heat treatment particularly to a higher austenitising treatment or lower tempering temperature. However, these measures lower the intrinsic ductility of the alloy steel.

ln steels of the present invention it is preferably to keep the carbon content in the range 0.22 to 0.26 percent by weight for optimum creep strength and creep ducility properties. Also the manganese and nickel contents should be controlled to within the stated ranges so that the combined manganese and nickel content in the steel is not greater than 1.5 percent by weight, in order to prevent the steel reverting from a preferred upper bainitic structure to an austenitic structure when tempered at 700 C. Furthermore the vanadium content is such as to ensure optimum creep strength for the steel, without embrittlement, by the formation of a vanadium carbide phase in the steel. As a result it is necessary to keep the contents of chromium and molybdenum, which are hardening elements, in the stated ranges so that the vanadium carbide is not detrimentally replaced by chromium and molybdenum carbides. The niobium and boron constituents in the steel jointly act to improve the steel properties and indeed the preferred upper bainitic structure can be obtained in forging sections up to 60 inches in diameter.- 3 k Thus with steels of the present invention the combination of constituents and heat treatment ensure an optimum combination of mechanical properties, creep strength and ductility. Such steels are highly suitable for the manufacture of large forgings for steam turbine rotors, particularly those having a mass equivalent to or greater than that of v 24 inch diameter cylinder when hardened. lt isalso suitable for large high pre's- While preferred embodiments have been described, it is to be understood that various modifications and changes may be made without departing from the spirit and scope of the invention. 1

What is claimed is:

1. Heat-resistant alloy steel consisting essentially of by weight from 0.20 to 0.30 percent carbon, from to 0.30 percent silicon, from 0.4 to L0 percent manganese, from 0.4 to 1.0 percent nickel, from 0.7 to 1.4 percent chromium, from 0.5 to 1.5 percent molybdenum, and from 0.25 to 0.40 percent vanadium, and further containing by weight from 0.03 to 0.15 percent niobium and from 0.002 to 0.010 percent boron to improve the hardenability, high temperature ductility and high temperature creep properties of the steel, the balance except for impurities and incidental constituents which include from 0 to 0.040 percent by weight sulfur and from 0 to 0.040 percent by weight phosphorus, being iron.

2. Heat-resistant alloy steel according to claim 1, containing by weight from 0.05 to 0.09 percent niobium and from 0.004 to 0.008 percent boron.

3. Heat-resistant alloy steel according to claim 1 in which the combined manganese and nickel content is not greater than 1.5 percent by weight.

4. Heat-resistant alloy steel according to claim 1, containing 0.22 to 0.26 percent carbon. 

2. Heat-resistant alloy steel according to claim 1, containing by weight from 0.05 to 0.09 percent niobium and from 0.004 to 0.008 percent boron.
 3. Heat-resistant alloy steel according to claim 1 in which the combined manganese and nickel content is not greater than 1.5 percent by weight.
 4. Heat-resistant alloy steel according to claim 1, containing 0.22 to 0.26 percent carbon. 